Rigid polyurethane (PU) foams as an interlayer for sandwich structures (shaped sandwich panels), and also the use thereof in the production of automotive interior trim, are known per se.
Sandwich structures for use as roof lining or pillar trim are nowadays usually produced by the cold-forming process from corresponding sandwich panels. In this process, the rigid PU foam panel is provided on both sides with a thermosetting adhesive and with reinforcing materials, such as mats or webs of glass fibre and/or natural fibre, and/or glass fibre rovings, and also with outer layers of paper, thermoplastic films and/or nonwoven fibre webs, and optionally decorative layers, and are subjected to deformation and pressing to a sandwich in a mould at temperatures of 100 to 150° C.
In the batch production of foam slabs (referred to as “box foaming”) there are differences in the cell sizes and physical properties, and particularly in the open-cell content, within a foam block. In the top and bottom regions, the cell sizes are smaller than in the middle. The cell sizes in the centre of a block are generally the greatest. With the use of isocyanates and polyol formulations, of the kind described in EP 0437787 A1, the cell sizes within a foam block differ significantly from one another depending on their location. The open-cell content and, less markedly, other physical properties too, likewise fluctuate. Particularly in the case of large-sized blocks with a volume of several cubic meters and heights of around 1 m, a markedly lower open-cell content is found in the lower region of the foam block.
In EP 0437787 A1, thermoplastically deformable rigid PU foams having densities of 25-30 kg/m3 are prepared by reaction of mixtures of diphenylmethane diisocyanates (MDI) and polyphenyl-polymethylene polyisocyanates (p-MDI) having an MDI content of 70 to 90 wt %, based on the isocyanate mixture, of which 12-30 wt % constitutes 2,4′-diphenylmethane diisocyanate, and with 10-30 wt %, based on the isocyanate mixture, of polyphenyl-polymethylene polyisocyanates (p-MDI) with a polyol mixture made up of 50 to 70 wt % (based on the polyol formulation) of a di- and/or trifunctional polyoxyalkylene polyol having a hydroxyl number of 28 to 600, 20 to 35 wt % of a difunctional phthalic acid polyester having a hydroxyl number of 150 to 440, 2 to 10 wt % of glycerol, 3.5 to 7 wt % of water, 0.3 to 1 wt % of an incorporable tertiary amine catalyst and 0.1 to 2 wt % of a silicone foam stabilizer.
These foams, however, exhibit considerable differences in foam structure depending on the location of the foam block. At the top and bottom, the foams are more finely cellular, i.e. the cell sizes are smaller, and in the middle the foams are more coarsely cellular, with the cell sizes therefore being larger. This is detrimental to the further processing to form, for instance, car roof linings. The foam block is cut into panels, which have different cell sizes in comparison to one another depending on their position within the foam block. This can lead to complications when these panels are processed to form sandwich structures.
It is an object of the present invention to prepare predominantly open-cell rigid PU foams with an extremely homogeneous cell structure. The panels produced therefrom are to be able to be pressed by the cold-forming process into sandwich structures, e.g. automotive interior trim and car roof linings.
It has been found that surprisingly, through the use of hexafunctional polyoxyalkylene polyols, the cell sizes within a foam block differ much less significantly from one another. At the same time, the physical properties within a foam block are also more homogeneous, and so on further processing to sandwich structures, for example, the metering of adhesive can be kept more constant and hence the reject rate can be reduced.